● the degree of selectivity available −methods may be selectivefor a small
number of analytes or specificfor only one;
● quality control and safety factors.Method validation Analytical methods must be shown to give reliable data, free from bias and suit-
able for the intended use. Most methods are multi-step procedures, and the
process of validation generally involves a stepwise approach in which optimized
experimental parameters are tested for robustness(ruggedness), that is sensi-
tivity to variations in the conditions, and sources of errors investigated.
A common approach is to start with the final measurement stage, using cali-
bration standards of known high purity for each analyte to establish the perfor-
mance characteristics of the detection system (i.e. specificity, range, quantitative
response (linearity), sensitivity, stability and reproducibility). Robustness in
terms of temperature, humidity and pressure variations would be included at
this stage, and a statistical assessment made of the reproducibility of repeated
identical measurements (replicates). The process is then extended backwards in
sequence through the preceding stages of the method, checking that the optimum
conditions and performance established for the final measurement on analyte
calibration standards remain valid throughout. Where this is not the case, new
conditions must be investigated by modification of the procedure and the process
repeated. A summary of this approach is shown in Figure 1in the form of a flow
diagram. At each stage, the results are assessed using appropriate statistical tests
(Section B) and compared for consistency with those of the previous stage. Where
unacceptable variations arise, changes to the procedure are implemented and the
assessment process repeated. The performance and robustness of the overall
method are finally tested with field trials in one or more routine analytical
laboratories before the method is considered to be fully validated.
8 Section A – The nature and scope of analytical chemistry